Bias trem... Again

[Chuck H]

The amp in question is running EL34's at 475Vp. There is a notable crossover distortion artifact that I would like to minimize. I don't have much experience with tremolo's. I've read that higher power amps don't do bias wiggle trem very well. But I don't entirely understand why. Even in an amp with lower Vp and a hotter bias, if you want the trem to "slam" it needs to drive the tubes into cutoff and subject the operating condition to the same cold bias state. Right? Any tips on minimizing the crossover garble at the cold end of the bias wiggle? I'm biased at about 70% right now and things are "acceptable" at best.

[soundmasterg]

The amp in question is running EL34's at 475Vp. There is a notable crossover distortion artifact that I would like to minimize. I don't have much experience with tremolo's. I've read that higher power amps don't do bias wiggle trem very well. But I don't entirely understand why. Even in an amp with lower Vp and a hotter bias, if you want the trem to "slam" it needs to drive the tubes into cutoff and subject the operating condition to the same cold bias state. Right? Any tips on minimizing the crossover garble at the cold end of the bias wiggle? I'm biased at about 70% right now and things are "acceptable" at best.

Which trem circuit are you using?

In my current build I've been messing around with the bias mod trem and am using the Vibrolux 6G11 oscillator. I tried it the CF and it was fantastic with lots of depth on a 6L6GC pair, but I need the half triode for other purposes, so I added a mosfet source follower circuit. I spent some time playing with the series resistor in between the source follower circuit and the intensity pot, and LED biased the oscillator, and found a happy medium that gives good depth but doesn't shut off too abruptly. That said, I like the CF better....it seems to have more depth and more life to it. I could be imagining things but thats what I'm hearing. Anyway, as for eliminating the crossover garble....well I'm still playing with it to do the same thing....

Greg

P.S. Without the CF or source follower, nothing I tried could get enough depth.

[Chuck H]

I think the need for a lower impedance source for the wiggle voltage has to do with the intensity pot and raw bias circuit impedance. I tried 6L6's in this amp (even though I don't like them as well for the "VOX" mode) fully expecting the crossover garble to diminish. But it sounded about the same. I've tried five different sets of power tubes in this amp. One set sounded best both tonally and in cooperation with the trem. Unfortunately those tubes were too mictophonic for long term use in the combo cabinet. But since there is nothing really different about the way those tubes biased up, and yet they seem to handle the trem better, I'm guessing that the "match" on those tubes may have been further off than the other sets. Perhaps some controlled degree of imbalance can improve things?!? I'll be experimenting with that when I can get back to the amp.

Thanks for responding. I've started a couple of threads that have gone unanswered! It was starting to seem like whenever I had a question it was nothing anyone knew about, had a good solution for or otherwise didn't want to touch on for some reason.

[thehoj]

I think the need for a lower impedance source for the wiggle voltage has to do with the intensity pot and raw bias circuit impedance. I tried 6L6's in this amp (even though I don't like them as well for the "VOX" mode) fully expecting the crossover garble to diminish. But it sounded about the same. I've tried five different sets of power tubes in this amp. One set sounded best both tonally and in cooperation with the trem. Unfortunately those tubes were too mictophonic for long term use in the combo cabinet. But since there is nothing really different about the way those tubes biased up, and yet they seem to handle the trem better, I'm guessing that the "match" on those tubes may have been further off than the other sets. Perhaps some controlled degree of imbalance can improve things?!? I'll be experimenting with that when I can get back to the amp.

Thanks for responding. I've started a couple of threads that have gone unanswered! It was starting to seem like whenever I had a question it was nothing anyone knew about, had a good solution for or otherwise didn't want to touch on for some reason.

Not sure if it helps you at all, but I recently modified a Traynor YVM-1 (PP EL34's at 440V) for guitar purposes, and used the bias wiggle trem circuit that Traynor used in their YSR-1, I'm very very happy with it.
Here's my full schematic http://s24.postimg.org/eoqm2uwfn/yvm...ack_w_trem.jpg

[cyclone]

I just put the traynor ygm3 style bias wiggle tremolo into my 20 watt fixed bias el84 amp design, it works well and has too much intensity, if anything. The thing is, with tremolo off, the bias is wherever the intensity pot is set, hotter at minimum intensity. So, I set the static bias with the intensity pot at min as per the traynor schem, so that the output tubes don't over dissipate at any setting, the tremolo just increases (negatively) the bias voltage to cut off the tubes. I'll probably change pot values or taper to get the control range I'm happy with, but set the bias with the control at the minimum intensity for safest and choppiest results